A currently used technique for improving performance of wireless communication systems, in particular in downlink direction, adds a delay diversity to the space and/or polarization diversity provided by transmitting antenna arrays. In other words, different elements in the array transmit differently delayed replicas of a same signal. In case of DDD technique, the different replicas undergo time-varying delays. At a receiver, the differently delayed replicas give rise to alternate constructive and destructive combinations.
A wireless communication system exploiting the DDD technique is disclosed for instance in WO 2006/037364 A.
Use of the DDD technique entails the provision of time-varying or tunable delay lines in the signal paths towards different antenna elements.
Assuming for sake of simplicity that the signals can be considered single-frequency signals, so that applying a time delay is equivalent to applying a phase shift, a delay line with length L introduces a phase shift φ=β·L, or a delay τ=dβ/dω, on the signal propagating through it, β being the propagation constant of the line and ω being the angular frequency. Thus, in order to vary the phase shift (or the delay), either β or L is to be varied. The most commonly used solution relies on a variation of β.
Several variable phase shifters based on the variation of β are known in the art, such lines generally relying upon the variation of the position of a dielectric member relative to a transmission line.
Variable phase shifters using microstrip transmission lines perturbed by dielectric elements are for instance illustrated in U.S. Pat. No. 6,075,424 A and U.S. Pat. No. 6,504,450 B2.
U.S. Pat. No. 6,075,424 discloses a phase shifter in which a dielectric slab is movable in the space between a transmission line and a ground plane. The slab has a width or a thickness or a dielectric constant that is variable from a leading edge to a trailing edge with reference to the direction of displacement, so that different relative positions of the slab and the line result in different values of the effective dielectric constant of the line and hence in different propagation velocities of the signal.
U.S. Pat. No. 6,504,450 discloses a phase shifter acting on a plurality of input signals. The shifter has a plurality of microstrip transmission lines shaped as concentric arcs of circumferences, and a semicircular dielectric member rotatable about an axis perpendicular to the plane of the transmission lines. The dielectric member, while rotating, covers increasing portions of each transmission line, thereby varying the phase shift induced by each of them.
In U.S. Patent Publication No. 2003/0042997 A1 and JP patent document 2001/068901 A are illustrated variable phase shifters implemented in rectangular waveguides.
U.S. Patent Publication No. 2003/0042997 Al discloses a phase shifter having an air-dielectric sandwich structure placed in a conventional rectangular waveguide. There, the dielectric constant of the structure, and hence the phase shift or the delay, is varied by varying the width of the air gap between a perturbing dielectric member and the waveguide walls.
JP patent document 2001/068901 A also discloses a phase shifter comprising a rectangular waveguide and a dielectric or metallic member partly inserted within the waveguide and movable with respect to the waveguide so that its insertion depth is changed.